Аннотация
Nine young infants were followed longitudinally from 4 to 15 months
of age. They performed multijoint reaching movements to a stationary
target presented at shoulder height. Time-position data of the hand,
shoulder, and elbow were collected using an optoelectronic measurement
system. In addition, we recorded electromyographic activity (EMG)
from arm extensors and flexors. This paper documents how control
problems of proximal torque generation may account for the segmented
hand paths seen during early reaching. Our analysis revealed the
following results: first, muscular impulse (integral of torque) increased
significantly between the ages of 20 (reaching onset) and 64 weeks.
That is, as infants got older they produced higher levels of mean
muscular flexor torque during reaching. Data were normalized by body
weight and movement time, so differences are not explained by anthropometric
changes or systematic variations in movement time. Second, while
adults produced solely flexor muscle torque to accomplish the task,
infants generated flexor and extensor muscle torque at shoulder and
elbow throughout a reach. At
reaching onset more than half of the trials revealed this latter kinetic
profile. Its frequency declined systematically as infants got older.
Third, we examined the pattern of muscle coordination in those trials
that exhibited elbow extensor muscle torque. We found that during
elbow extension
coactivation of flexor and extensor muscles was the predominant pattern
in 67% of the trials. This pattern was notably absent in comparable
adult reaching movements. Fourth, fluctuations in force generation,
as measured by the rate of change of total torque (NET) and muscular
torque (MUS), were more frequent in early reaching (2028 weeks)
than in the older cohort (5264 weeks), indicating that muscular
torque production became
increasingly smoother and task-efficient. Our data demonstrate that
young infants have problems in generating smooth profiles of proximal
joint torques. One possible reason for this imprecision in infant
force control is their inexperience in predicting the magnitude and
direction of external forces. That infants learned to consider external
forces is documented by their increasing reliance on these forces
when performing voluntary elbow
extensions. The patterns of muscle coordination underlying active
elbow extensions were basically the same as during the prereaching
phase, indicating that the formation of functional synergies is based
on a basal repertoire of innervation patterns already observable
in very early, spontaneous movements.
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